Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature. Generally, these processes have in common the steps of employing an electrophotographic element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a record of the image.
One type of unitary photoconductive element particularly useful in electrophotography is generally produced in a multilayer structure. Such an element is prepared, for example, by coating one or more layers of an insulating photoconductive composition onto a support which previously has been overcoated with a layer of electrically conducting material. In addition, a polymeric interlayer is often interposed between the conducting material and photoconductive composition of such unitary multilayer elements to provide adhesion and/or to serve as an electrical barrier layer between the conducting material and the photoconductive composition.
Representative publications which disclose various polymeric materials which may be employed as interlayers for use in a unitary multilayer element of the type described immediately hereinabove are set forth, for example, in U.S. Pat. No. 3,640,708 issued Feb. 8, 1972, U.S. Pat. No. 3,438,773, issued Apr. 15, 1969; U.S. Pat. No. 3,745,005 issued July 10, 1973; and U.S. Pat. No. 3,932,179 issued Jan. 13, 1976.
As indicated in the above patent publications, one particularly useful component in such polymeric interlayers is a copolymer such as a terpolymer of tetrapolymer which is hydrophobic and which has a substantial number of repeating units derived from a carboxylic acid group such as itaconic acid, acrylic acid, and the like, and/or a substantial number of repeating units derived from vinylidene chloride. Although hydrophobic terpolymers and tetrapolymers prepared containing the above-described repeating units have been found to provide good adhesive properties for use in a unitary multilayer photoconductive element as described hereinabove, it has recently been determined that these hydrophobic terpolymer and tetrapolymer materials can seriously interfere with the electrical characteristics and operating properties of multilayer photoconductive layers. In particular, it has been found that the above-described polymeric materials which contain acid components, such as itaconic or acrylic acid, or units derived from a monomer such as vinylidene chloride which is subject to degradation to form an acid (i.e., hydrochloric acid), can seriously impair the electrical characteristics of the photoconductive composition associated with said multilayer photoconductive element.
In addition to the foregoing patent publications, other patent publications such as U.S. Pat. No. 3,647,432 issued Mar. 7, 1972 and U.S. Pat. No. 3,765,884 issued Oct. 16, 1973 have described compositions composed of certain organic photoconductor or sensitizer materials admixed with any one of various binder materials. Included within the extensive listing of useful such binder materials are polycondensate polymers such as a polyester of ethylene glycol, neopentyl glycol, terephthalic acid and isophthalic acid. Although the aforementioned polyester can be employed as the binder material of a photoconductive insulating composition, it has been found that this polyester when present as the sole binder component of, for example, certain homogeneous organic photoconductive insulating compositions, can interfere with the electrical operation of the resultant photoconductive composition such that it is incapable of readily accepting an initial electrostatic charge of a magnitude within the desired operating ranges of such photoconductive compositions, i.e., 600 volts or more.